Simplify Select interaction and prepare for programmatic feature selection without interaction

As discussed in the weekly hangouts this and last week, this pull request simplifies the Select interaction by not using a temporary layer any more. Further simplifications are accomplished by providing a `setRenderIntent` method on the feature (instead of the layer), and adding a `getFeatures` method to the layer (to be moved to the source when we move the feature cache from the layer to the source).

After this change, if we decide to expose `setRenderIntent` and `getFeatures`, applications will be able to select features without using a select interaction.